Charter-school teachers and other staff members in Chicago went on strike Tuesday, the first such labor action by educators at a charter-school network in the country.

The striking teachers, paraprofessionals and support staff work at 15 charter schools operated by the UNO/Acero charter network and have been negotiating for improvements on a number of issues, including class size (there are now 32 students in each class), the need for more special education staff and raises for underpaid employees.

The teachers and staff, who are represented by the Chicago Teachers Union, broke off negotiations after threatening a strike if no progress was made at the bargaining table.

The action is the latest teacher strike this year, which began with walkouts by public school teachers in mostly Republican-led states, including West Virginia and Oklahoma. Educators demanded higher wages and more resources for ailing schools.

This is, however, the first time that educators at a charter network have gone on strike after negotiations failed. Charter schools are publicly funded but privately operated, and most are not unionized. The National Alliance for Public Charter Schools reported that in 2016-2017, 11.3 percent of charter schools were participating in collective bargaining agreements with teachers unions.

In California this year, teachers at the state’s largest virtual charter school, the for-profit California Virtual Academies, won a landmark union contract only after threatening to strike. A pact securing a pay raise and due-process rights was won from the operator of those schools, Virginia-based education giant K12 Inc., after years of organizing and negotiations — and, ultimately, the strike threat.

The Chicago Teachers Union says that Acero teachers are paid up to $13,000 less than teachers in the traditional public school district.

Teacher activism has been robust in Chicago, where a 2012 teacher walkout won the support of the community and may have helped pave the way for the strikes of 2018.

The Illinois Network of Charter Schools released a statement saying that it was “disappointed with the breakdown of contract negotiations, prompting charter public school teachers who are unionized to strike.” It also said that the organization has “always honored the contribution of charter teachers, who work daily to create safe and productive learning environments for all students,” but it went on to attack the Chicago Teachers Union for failing to partner with the charter community.